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It seems more than a few Kiwi boaties identified with the Practical Boating story "Driveline Diagnostics" in the November 2012 issue of Boating, which set out to help the owners of inboard, shaft-driven boats eliminate excessive vibration.

A number of readers have since approached Mark Power, the managing director of Henleys Propellers & Marine who was quoted extensively in the article, for further information.

The feedback has prompted Boating to print some bonus technical advice from Power here.

Driveline Diagnostics

The engine is the only variable (easily moveable) component in the driveline. Henleys Propellers & Marine managing director Mark Power advises vibration sleuths to begin at the front of the system (while the vessel is in the water) and work aft if required. A quick preliminary check is to move the prop shaft by hand - it should rotate freely. If it doesn't or it binds, alarm bells should ring.

Once on the hard a piece of fishing line, piano wire or laser is the method for checking the alignment of stern tube and strut bearings to the centre of the gearbox output flange. The prop shaft is removed and the line is run down the centre of the front end of the stern tube (inside the boat) through the centre of the aft end of the stern tube (both stern tube references are fixed in the vessel) to a solid post behind the strut. A set of inside callipers is used to check for a uniform distance between the line and inner surfaces of the bearings (if not worn) or bearing housing ID. This will establish if the strut is in alignment with the stern tube. If OK the next step is to ensure that the line forward to the gearbox output flange has remained in the centre to establish if the engine is in alignment with the shaft centre line. Adjustments can be made on the hard but final flange to flange alignment must be done once the vessel is in the water.

"Bearings need water for cooling and lubrication - reducing water access severely shortens their life. If bearings are installed correctly and operate free of foreign matter they should last for the life of an average pleasure craft (100 hours PA)," Power says. Commercial operators achieve 7000+ hours from their correctly installed bearings, Power said. "Expected Thordon bearing life in a water cooled cruise ship is 25 years of continuous use!"

Most boat owners are aware that gearbox and prop shaft flanges need to mate perfectly. A discrepancy of one or two thou is desirable - the maximum installed tolerance is three - thou. It's within the scope of the average DIYer - armed with a feeler gauge and lots of patience - to make the miniscule adjustments at the engine mounts. However, Power insists: "This is a job for an engineer, not a carpenter". He recommends that one uses a professional to ensure this critical piece of the puzzle is achieved and maintained.

"A common DIY fault is ignoring the offset issue. The two flanges might be perfectly parallel, but if they aren't centred they'll create shaft oscillation which in turn places excessive loads on the gearbox output bearing, flex coupling, shaft seal, bearings and shaft."

If an engine is rubber mounted, alignment should be checked every 100 hours. Using a static Urethane-based mount system one should check alignment every 500 hours.

"More importantly mounts should be correctly sized," Power says. "And sizing is not just a function of the engine/gearbox weight - the equation includes other variables such as the gearbox's reduction ratio, diameter and pitch of the propeller and the intended purpose of vessel.

"The individual components may be of sound design and build but in a combined format in relation to ones vessel, can produce an unforeseen problem. Car manufacturers etc. spend millions of dollars on R&D to prevent resonance - they can afford too!"

Unfortunately there are not many designers, builders or owners of pleasure craft that can afford the luxury of such R&D on what is mostly low volume builds. That's where the team at Henleys can help.